AlGaN/GaN high-electron-mobility transistors (HEMTs) on 4 in. Si were fabricated by inserting 3 nm of Al metal as a gate prior to the deposition of Pd/Ti/Au. The increase of drain current (IDS max) density and decrease of extrinsic transconductance (gm max) have been observed in the Al-gated AlGaN/GaN HEMTs. The increase of ID max is due to the increase of two-dimensional electron gas sheet carrier density, which was confirmed by capacitance-voltage (C-V) measurements. Moreover, the Al layer inserted-gate HEMT exhibited negative threshold voltage (Vth) shift. The Al and AlGaN interface shows Al-based oxide layer which was confirmed by Auger electron spectrum and x-ray photoelectron spectrum.